Dialectical behaviour therapy (DBT) is an evidence-based therapy for people with borderline personality disorder (BPD). Past research has identified behavioural changes indicating improved functioning for people who undergo DBT. To date, however, there has been little research investigating the underlying mechanism of change. The present study utilised a between-subjects design and self-report questionnaires of Self-Control and the five factor model of personality and drew participants from a metropolitan DBT program. We found that pre-treatment participants were significantly lower on Self-Control, Agreeableness and Conscientiousness when compared to both the post-treatment assessment and the norms for each questionnaire. Neuroticism was significantly higher both before and after treatment when compared to the norms. These findings suggest that Self-Control may play a role in both the presentation of this disorder and the effect of DBT. High levels of Neuroticism lend weight to the Linehan biosocial model of BPD development.